The FBI, which now is leading the investigation, believes the gunman used the identification of a man who used to work at Navy Yard... The ID was found next to his body in Building 197, where most of the victims were shot on the third and fourth floors.

NBC News correspondent Pete Williams says all indications show it was a lone gunman who was responsible for today's shooting. Williams says the white male that DC police said earlier they were looking for as a possible additional gunman has been cleared and is not a suspect.

According to the arrest report, Alexis' upstairs neighbor called police after a hearing a pop and seeing a cloud of dust come up from her floor. The woman called police and said a bullet ripped through her floor and ceiling from the apartment downstairs. Fort Worth police knocked on her neighbor's door, but he didn't answer. As police were about to enter the apartment through force, the man, identified as Alexis, came outside willingly.

Alexis told officers he had been cleaning the weapon when it slipped and and that he accidentally pulled the trigger. He said he didn't check on his neighbor because he couldn't see light through the hole in his ceiling and that he assumed the bullet hadn't passed through her floor. He also said he thought his neighbors would assume the sound of the gunshot was a firecracker. According to police, Alexis told them he didn't answer the door initially because he assumed it was his upstairs neighbor, with whom he shared a contentious relationship.

The Tarrant County district attorney's office never filed charges against Alexis.